1. Field of the Invention
The present invention relates to voltage source switching for electronic devices and, more particularly, to an improved voltage source switching circuit for use in electronic devices.
2. Description of the Related Art
Electronic devices, such as portable computing devices, have long been powered by alternative power sources. Typically, an electronic device can receive power from a main power source and an auxiliary power source. By way of example, in the case of a portable computing device, the main power source is AC power supplied by an AC outlet, and the auxiliary power source is DC power supplied by a battery. The auxiliary power source, e.g., the battery, is often employed as an alternative an/or backup power supply when the main power source is unavailable. The auxiliary power source, e.g., the battery, may also be used when the electronic device is turned off to maintain certain circuits such as memory cells that require small amount of power to maintain their data.
Conventionally, switching circuits have been used to facilitate switching between different power sources. FIG. 1 depicts a conventional line switching circuit 102 for use in a portable computing device. The conventional line switching circuit 102 facilitates switching between a main power source 104 and a battery 108. The main power source 104 is a primary power source and the battery 108 is an auxiliary power source. The main power source 104 can be connected to an AC power line 106, typically via an AC power outlet. The battery 108 functions as a backup or alternative source of power. A charge circuit 114 is often provided to charge the battery 108 using power from the AC power source 104.
The conventional line switching circuit 102 operates to couple either the main power source 104 or the battery 108 to an output power line 110. The output power line 110 is often used to provide power internal to the electronic device or to a peripheral circuit board 112. When the AC power line 106 is active and the electronic device is operating, it is desirable to supply power to the output power line 110 from the main power source 104. In other situations such as unavailability of AC power from the AC Power line 106 or when the electronic device is powered off, it may be desirable to supply power to the output power line 110 from the battery 108.
In such situations, to facilitate the switching between the main power supply 104 and the battery 108, the conventional line switching circuit 102 uses two diodes 114 and 116. The diode 114 is connected between the battery 108 and the output power line 110. The diode 116 is connected between the main power source 104 and the output power line 110. The switching provided by the diodes 114 and 116 is such that the power supply coupled to the output power line is either the main power supply 104 or the battery 108 depending on whichever offers the greater voltage level. For example, when the voltage at the main power source 104 is available and has a value of about 5.2 Volts and the battery 108 has a voltage of about 5 Volts, the conventional line switching circuit 102 would couple the main power source 104 to the output power line 110 because the main power source 104 has the higher voltage value. To elaborate, in this situation, with diodes 114 and 116 having identical characteristics, diode not switch-on since it would be biased below its turn-on voltage. In another situation, when the main power source 104 is not available, the voltage level for main power source 104 is dropped to about zero (0) Volts (or for some reason drops below the voltage of the battery 108), the diode 114 would switch-on and couple the battery 108 to the output power line 110 (and the diode 116 would be switched off).
One major disadvantage with the conventional line switching circuit 102 is that there is a significant voltage drop across the diodes 114 and 116. That is, voltage level for output power line 110 is significantly lower than the voltage level at the selected power source (i.e., either main power supply 104 or battery 108) due to the voltage drops across the associated diodes 114 and 116. Typically, the voltage drop is about 0.6 Volts or higher when silicon diodes are used. By way of example, if the main power supply 104 has a voltage level about 5.0 Volts, then the voltage on the output power line 110 would be about 4.4 Volts or lower when the main power supply 104 is utilized. Similarly, when the battery 108 is utilized, the battery 108 has a voltage level about 5.0 Volts because of the voltage drop for the diode 114 the output power line 110 would be about 4.4 Volts or less. To reduce voltage drops across the diodes 114 and 116, schottky diodes can be used. Schottky diodes have a lower voltage drop (about 0.4 volts) than silicon diodes (about 0.6 volts), however, schottky diodes are more expensive components than silicon diodes. It should also be noted that germanium diodes may offer a lower voltage drop than schottky diodes (about 0.2 Volts), however, they have higher reverse current leakage which makes them undesirable for power switching applications.
The effect of the voltage drop across the diodes 114 and 116 is to reduce the voltage appearing on the output power line 110. Typically, the output power line 110 is used to supply power to the peripheral circuit board 112. However, the peripheral circuit board 112 is manufactured with certain tolerances and the reduced voltage supplied to the peripheral circuit board 112 as a result of the voltage drops can lead to breach of tolerances and thus unstable conditions. While use of schottky diodes are of some hell), even using schottky diodes to implement a convention line switching circuit cannot satisfactorily address the problems associated with the undesired voltage drops, which are becoming more severe as operating voltages drop (e.g., from 5 Volts to 3 Volts) primarily for power conservation reasons. For example, if the tolerance level for the peripheral circuit board 112 is 10% and the main power supply 104 is on the order of 5 Volts, the peripheral circuit board 112 can function properly as long as the voltages provided to the peripheral circuit board 112 do not fluctuate more than 10%. (i.e., accepted range would be 4.5 to 5.5 volts). In this situation, even if the tolerance level for the main power supply 104 itself is 5% (i.e., power supply 104 is guaranteed to be output voltages between 4.75 volts to 5.25 Volts), there can only be a voltage drop up to 0.25 Volts across diodes 114 and 116 in order to remain within tolerance (e.g., 4.75-4.50=0.25). Recall, even using schottky diodes, the conventional line switching circuit 102 could yield a voltage drop of about 0.4 Volts. Therefore, the voltage levels provided to the peripheral circuit board 112 could fall below accepted levels.
To address the problem of voltage drop levels associated with diodes, a switching regulator could be utilized to boost voltages output from the power supplies to compensate for the voltage drop due to the diodes. However, using a switching regulator, especially for boosting DC voltages is an inefficient, costly, and complicated solution.
In the view of the forgoing, there is a need for the voltage source switching circuit capable of outputting voltages at a reduced voltage drop level.